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Ab initio SCF assignment of vibrational spectra of nitrosomethane
Volume 99, number 4
CHEMICAL.PHYSICS UEmERS
12 August 1983
AB INITIO SCF ASSKNMENT OF VIBRATIONAL SPECTRA OF NIT~OSOMETH~E J.P. DOGNON, C. POUCHAN and A. DARGELOS Labomtoire de C%imie Structurale (ERA 895). IURS. Universitt? de Pau, Avenue Louis Sallenove, 64000 Pau. France Received 14 Xiarcb 1983; in final form 16 May 1983
The force constants of nitrosomethane obtained from an SCF wavefunction using a 4-31G basis set are compared with experimental data, which are critically examined_A calculation of intensities was also performed. The theoretical results
suggest a new interpretation of the infrared spectrum.
1. Introduction There are only a limited number
of infrared
spectro-
scopic studies of nitrosomethane in the literature [I3]_ This is due to esperimental problems of obtaining the monomer from the thermal dissociation of the
dimer. Work in the vapor phase is Iimited by incomplete dissociation. isomerizatior; of the dimers and then rapid thermal rearrangement of the monomer into formaldosime [ I]. Thus. the most complete infrared spectrum yet obtained with nitrosomethane resulted from an analysis in an argon matrix [4] at low temperature (20 K)_ This study is insufficient for unequivocally
assigning
all the bands in tile spectrum. In no case does it enable a potential function to be proposed based only on experimental data [4,5] which can assist in the interpretation of the vibrational spectrum_ In the present work, we used an ab initio SCF catculation to propose a complete force field, and an infrared intensity calculation. This leads to the a priori construction of the nitrosomethane vibrational spectrum and thus to remove doubt still remaining in the assignment of the vibrational spectrum.
tions, utilizing a double-zeta basis set in the valence region (4-3 1G). This choice was dictated by the fact that this type of basis set enables these vibrational spectroscopic figures to be obtained with a reasonable approxhnation and in a relatively economic manner [6]. The geometric structure used (table 2) to represent the nitrosomethane molecule in its equilibrium configuration is that optimized by Adeney et al. [73 in the 4-3 1G basis set. The vibrational analysis was performed with the formalism of Wilson et al. [8] from data deduced from our cafculations. It was complemented by a theoretical determination of the integrated absorption intensities A, for each mode Q=_ These intensities are proportional to (ap/aQ,)’ in the hypothesis of electrical and mechanical harmonicity- The principle of their calculation requires knowledge of the mode Qn and of the dipole moment gradient in relation to this mode [8-IO]. it may be summarized as follows. In tk basis set of internal coordinates with S symmetry
3. Calculation conditions The study of the force field and of dipole moment derivatives was undertaken in the space of internal group coordinates (table I) with an initio SCF calcula316
* The variations of the cartesian coordinates used to calculate +/X3 are estimated in such a manner that the direction of the principal axis of the inertia teusor should be invariant. In these conditions the rotational contribution to the infrared intensities will be canceled [ 1 1 ] _
0 009~2614/83/0000-0000/$03.00
0 1983 North-Holland
CHEMICAL
Volume 99. number 4 Table 1 internal and symmetry
coordinates
-12 August 1983
PHYSICS LETTERS
for nitrosomethane
Assignment A’ symmetry
&
Ar
C-N
S,
AI
s3
3-‘“(Ad1
s4
6-‘R(2Ad,
N-O stretch CH3 sym. stretch CH3 asym. stretch CNO bend CH3 sym. deformation CH3 asym. deformation
%
S8
Ar 61n(Af31 + A& + AD3 - A~12 - Aa13 - A&23) 6-1n(2A~23 - Acrl2 - Acq3) 6-ln(2ADB1 - AS2 - A@3)
Sg
2-‘n(Adl
SlO
2e1R(Aa13
Sll
2-?A&
Sl2
AT a)
s6 S7
A” symmetry
a)AT = $(AT~,CNO
Ad,) - Adz - Ad3)
+ Ad2 +
Aq2)
torsion
[7]
Microwave [ 5 ]
l-471 1.198
1.480
C-Hr
1.079
1.094
C-H1
1.080
1.211
YN-O
VCH3s ?H3as 6CNO ‘CH3s 6CH3as ‘KH3 “CH3as
‘CH3as ‘_LCH3 T
with
ap/asi = (,=xq, (artr/asi12)1'2I ,
A, is expressed in P mol-l g’/2 cm-1 _
cmm2 and ap/aQ,
in esu
1.094
LCNO
114.9
113.2
LHICN
111.2
111.0
LHzCN
107.4
107.2
3. Results and discussion 3_1. Force constants The force constants obtained with the double-zetatype ab initio basis sets are generally well reflected. In
from which
A, = 0.305
?-N
+ AT~~CNO + AT~~CNO), I’ = dihedral.
4-31G
N-O
CH3 rock (out of plane)
- AP,)
Table 2 Calculated and observed structural parameters of nitrosomethane (bond length, .4; angle, deg)
C-N
CH3 rock (m plane)
CH3 asym. stretch CH3 asym. deformation
-Ad,) -
stretch
(F (W’Si)Lin)2,
order to account for both correlation and anharmonicit) effects, however, is it necessary to apply respective corrections of 10 and 20% to the valence diagonal and angular deformation terms [6] _ The crossed terms of the 317
CHEMICAL PHYSICS LETTERS
Volume 99, number 4
potential function were not modified. The force constants for the nitrosomethane molecule thus obtained are shown in table 3. As a result of the small number of experimental data, it was not possible to compare directly these values to a generalized valence force field. On the other hand, the Tzbie 3 Computed force field of nitrosomethane (in mdvn X-‘, mdsn rad-’ and mdvn ii rad-’ for z&etching, stret~hin~ben~mg and bending force constants respectively) Assignment
4-31G
4.040 0.916 0.169 -0.013 0.562 0.452 0.099 0.023 13.042 0.039 0.073 o-705 0.044 0.002 -0.106
_4ssigmnent
4-31G
5_3J5 -0.162 -0.014 0.183 0.142 1.626 0.035 0.034 -0.271 0.598 0.003 0.009 0.562 0.023
5.366 -0.108 -0.018 -0.091 -0.024 -0.009
4.997 -0.094 0.229 0.092 0.494 -0.329 -0.009
0.699
0.759 -0.006
0.014
12 August 1983
partial results of Turner and Cox 15] in the hypothesis of a simplified force field, as well as those proposed by Barnes et al. [4] for the study of the CNO group, may be valuable for discussionConcerning the main force constants of the CN and CNO vibrators, the results obtained (FCN = 4.04 mdyn A-’ and FCNo= I.626 mdyn A-f) are in excellent agree ment with those reported by Barnes et al. [4] (FCN =r. 3.80 mdyn A-’ and FCNo = 1.6 mdyn A-l) and are somewhat different from those reported by Turner and Cox [S] (FCN = 3.5 1 mdyn ,&-I and FCNo = 2.04 mdyn A-l)_ These values may also be compared to those determined by Pouchan et al. [ 121 for formaldoxime (FEND = 1.66 mdyn A-I) and by Shurvell et al. { 135 for tri~uoronitrosomethane (FC- = 3.616 mdyn a-t and In the latter case, the deFCNO = I.29 mdyn a-l). creased force constants may be explained by the elec~roneg3tivlty of the fluorine atom. Concerning the force constant of the NO vibrator, the calculated value (pNo = 13.042 mdyn A-1) is quite different from those calculated by Barnes et al. [4] (IO.80 mdyn A-l) and by Turner and Cox [S] (9.87 mdyn A-‘) for CHsNO. it is also different from that reported by Shurvell et al. 1131 (10.10 mdyn A-1) for the trifluorinated homologue. This disagreement, already reported for the formaldoxime molecule [ 12], is due primarily to the poor description of the atomic orbitals of the N atom in the 4-3 1G basis set [6]_ It should nonetheless be noted that an iterative catculalion on Fho based on the calcutated force field and reliable experimental frequencies generated a value of 10.88 mdyn A-’ for this constant, identical to that proposed by Barnes et al. [4]. For the other diagonal force constants associated with the movements of the methyl group, we note a satisfying agreement between our values and those calculated for the analogous vibrators of methyl formate [ 141 and acetaldehyde 1151, a molecule which is isoelectronic with nitrosomethaneThe constant F, of the torsional mode could not be experimentally determined. Our calculations result in a low value of 0.014 mdyn A-l rade2, included between that calculated for acetaldehyde (IS] (F,= 0.037 mdyn A-l) or methyl formate [ 141 (F,= 0.021mdyn A-l) and that determined for tri~uoronitrosomethane [ 131 (F,= 0.004mdyn A-l). All the crossed terms of the potential function
CHEMICAL
Volume 99, number 4
based on the analysis of the potential energy distribution, is shown in table 41 This spectroscopic study was complemented by the calculation of integrated absorption intensities. It enabled the infrared spectrum of nitrosomethane to be plotted theoretically and a priori (fig. 1). Several comments may be made concerning the comparison of our results and those of Barnes et al[4], obtained in an argon matrix. (i) All the calculated modes develop preferentially along one group coordinate, facilitating the assignment of the spectrum. (ii) The theoretical sequence of band positioning agrees with the experimental sequence. Even in the relatively restricted spectral range of methyl group valence vibrations, our calculations confirm the fact that the vCH,mode with A” symmetry is located between the vCHas and vCHa% modes with A’ symmetry and has a higher intensity that the latter two. (iii) Concerning the modes of the CHO group itself, it should be noted that there is excellent agreement between the wavenumbers calculated at 1564,896 and 541 cm-l to describe the movements of vNO, vcN and sCNo and the corresponding frequencies (1549,870 and 574 cm-r) observed in the matrix. This concordance in band position is corroborated by examining their intensity.
could not be obtained unambiguously by experimental means as a result of the insufficiency and imprecision of the latter. We may note the difference between the calculated value for FCN/N~ at 0916 mdyn A-l and the value proposed by Turner et al. [5] (1.5 1 mdyn A-l). In addition, there is a complete disagreement between our calculated value of 0.705 mdyn A-l for FN,,c.O and the negative value (-0.3 1 mdyn A-l) reported by Turner et al. [5] _ In the latter case, our results are consistent with those obtained with the homologous terms of the formaldoxime molecule [ 12]_ Similarly, there is perfect agreement between the value calculated for FCNIcNO (0.562 mdyn) and the corresponding values proposed in the case of fomraldoxime (0.526 mdyn) and trifluoronitrosomethane 1133 (0.3 16 mdyn). Among the other interaction force constants, supposed null in all experimental approaches, we noted relatively high values for the terms FCN16CHss, F CHO/r,,CH3 J FCH,as/r1CH3 and FsCH&lCH~3.2
Vibrational spectrum
The vibrational spectrum of nitrosomethane was constructed from frequencies calculated with the formalism of Wilson et al. [S]. The assignment performed, Table 4 Vibrational frequencies (cm-*)
calculated for nitrosomethane Yobs
A’
A"
12 August 1983
PHYSICS LETTERS
“cdc
a)
DEP b,
2991
3151
~CH,&92),
2901
3028
1549
1563
1410
1496
%H3s (‘l)- “CH3as cg) “NO (85 j, 5 &gas _ c6)* ‘CH$[ (51, VCN (21, SC,,0 6CH3as (841, +k:+ (61, YN(-J (8)
1348
1368
+H3s
967
1174
‘CH3n (701, SC,,0
870
896
VCN (86). pCH+s
574
541
&-NO (77),rCHa,,
2955
3052.
‘)CH3”s
1410
1440
sCHgas (86).‘CH3~
(13)
916
907
rCH31 (13),6CH,as
(36)
144
166
7 (831, “CH+
pCH3s
(8) (2)
cgl), 6CH3ar (5). PNO (2) (121, VCN (10). “NO (41, +H3s
(3)
(4). TCH311 t7) (22)
(100)
(1 l), 6CH+
(3). rlCH,
(3)
a) Corrected force COnSWitS (See text) - FNO = 10.88 mdyn 8;’ _ b) DEP calculated using corrected force field.
319
CHEhlICAL
full
0
1111
Wave
number
The theoretical study of the other modes is an indispensible support for the experimental study insofar as the ldtter alone cannot (i) distinguish the bands of 6~11~~ deformation modes with A’ and A” symmetry in the region of 1400 to 1.500 cmS1; (ii) subsequently assign a slightly intense but very wide band centered at around 1162 cm_1 ; (iii) finally, define the frequency of the torsional mode. The a priori study of the infrared spectnnn of nitrosomethane removes all ambiguity concerning these three points, primarily involving the modes associated with the methyl group. Our results seem to be valid, since the other two movements, SC& (A’) and rCH3 (A”), creating two bands with high and low intensity at I348 and 9 16 cm-l are shown perfectly by our ab initio calculations at 1368 and 907 cm-l_ In the region between 1400 and 1500 cm-l, our calculations predict two vibrations with different intensi-
width
at
fill
4511
Fig. 1. IR spectrum calculated
320
1Z AUfgISI lYiS5
PHYSICS LETTERS
half
maximum -1 20 cm
:
2211
(cm-‘)
for qitrosomethane.
ties, at 1496 and 1441 cm-l, assigned respectively to the A’ and A” asymmetric deformation modes of the methyl group. The difference between the two bands is greater than that observed or calculated for the same vibrations in acetaldehyde [15] and methyl formate [ 141. This enables us to predict the existence of two distinct bands which were not differentiated in the experimental analysis of Barnes et al. [4] as a result of the numerous lines arising from incomplete dimer elimination. The analysis of our calculations subsequently enables us to affirm that the wide band observed but not assigned by Barnes et al. [4] at 1162 cm-l is most probably attributable to the rCHJ rocking mode with A’ symmetry_ This vibration, calculated at 1175 cm-l, presents a small calculated intensity and may be compared to the frequencies chosen to describe this same mode in acetaldehyde [IS] (1114 cm-l) and in methyl
Volume 99,
number4
CHEMICAL PHYSICS LE-JTERS
formate [I43 (Ii68 cm-l). In these conditions, it is thus probable that the wavenumber of 967 cm-l adopted by Barnes et al. [4] to describe rocking movement is in error. The existence of this band is probably due to the presence of the trans dimer which absorbs to a great extent in this region. Finally, our calculations indicate that the torsional mode of the methyl group should appear at 166 cm-l in an experimentally unexplored spectral region. This value is in agreement with that estimated at 144 cm-l by Turner and Cox [S], based on the study of satellite lines in a microwave spectrum.
[I] W. Liittke, Z. Elektrochem. 61 (1957) 302. [2] R.N. Dixon and H.W. Kroto, Proc. Roy. Sot. A283 (1964) 423. [3] N.P. Emsting, J. Pfab and J. Riimelt, J. Chem. Sot. Faraday II 74 (1978) 2286.
I2 August 1983
[4] A.J. Barnes, HE. Hallam. S. Waring and J-R. Armstrong, J. Chem. Sot. Faraday II 72 (1976)-l_ [5] P-H. Turner and AP. Cox, f_ Chem. Sot. Faraday II 71 (1978) 533. [6] C_ Pouchan, A. Dargelos and M. Chaillet, J. Chlm. Phys. 75 (1978) 595. [7] P-D. Adeney, WJ_ Bouma, L_ Radom and W-R. Rodwell. J. Am. Chem. Sot. 12 (1980) 4069. [S] E-B. Wilson, J-C. Decius and P-C. Cross, Molecular vibrations (McGraw-Hill, New Y&k, 1955). [9] G.M. Barrow, Molecular spectroscopy (McGraw-Hill, New York, 1963). [lo] J. Overend, Infrared spectroscopy (Elsevier, Amsterdam, 1963). [ 1 l] TX Lakdar, M. Suard. E. Taillandier and G. Berthier, hiol. Phys. 36 (1978) 509_ [ 121 C. Pouchan, D. Liotard, A. Dargelos and M. ChaiUet, J. Ch-b-n. Phys. 11 (1976) 1046. [ 131 H-F. Shurvell, S-C. Dass and RD. Gordon, Can. J_ Chem. 52 (1974) 3149. [ 141 E.B. Marmar, C. Pouchan, A. Dargelos and M. Chaillet, J. Mol. StNCt. 57 (1979) 189. [IS] H. Hollenstein and Hs.H. Giinthard, Spectrochim. Acta 27A (1971) 2027.
321
CHEMICAL.PHYSICS UEmERS
12 August 1983
AB INITIO SCF ASSKNMENT OF VIBRATIONAL SPECTRA OF NIT~OSOMETH~E J.P. DOGNON, C. POUCHAN and A. DARGELOS Labomtoire de C%imie Structurale (ERA 895). IURS. Universitt? de Pau, Avenue Louis Sallenove, 64000 Pau. France Received 14 Xiarcb 1983; in final form 16 May 1983
The force constants of nitrosomethane obtained from an SCF wavefunction using a 4-31G basis set are compared with experimental data, which are critically examined_A calculation of intensities was also performed. The theoretical results
suggest a new interpretation of the infrared spectrum.
1. Introduction There are only a limited number
of infrared
spectro-
scopic studies of nitrosomethane in the literature [I3]_ This is due to esperimental problems of obtaining the monomer from the thermal dissociation of the
dimer. Work in the vapor phase is Iimited by incomplete dissociation. isomerizatior; of the dimers and then rapid thermal rearrangement of the monomer into formaldosime [ I]. Thus. the most complete infrared spectrum yet obtained with nitrosomethane resulted from an analysis in an argon matrix [4] at low temperature (20 K)_ This study is insufficient for unequivocally
assigning
all the bands in tile spectrum. In no case does it enable a potential function to be proposed based only on experimental data [4,5] which can assist in the interpretation of the vibrational spectrum_ In the present work, we used an ab initio SCF catculation to propose a complete force field, and an infrared intensity calculation. This leads to the a priori construction of the nitrosomethane vibrational spectrum and thus to remove doubt still remaining in the assignment of the vibrational spectrum.
tions, utilizing a double-zeta basis set in the valence region (4-3 1G). This choice was dictated by the fact that this type of basis set enables these vibrational spectroscopic figures to be obtained with a reasonable approxhnation and in a relatively economic manner [6]. The geometric structure used (table 2) to represent the nitrosomethane molecule in its equilibrium configuration is that optimized by Adeney et al. [73 in the 4-3 1G basis set. The vibrational analysis was performed with the formalism of Wilson et al. [8] from data deduced from our cafculations. It was complemented by a theoretical determination of the integrated absorption intensities A, for each mode Q=_ These intensities are proportional to (ap/aQ,)’ in the hypothesis of electrical and mechanical harmonicity- The principle of their calculation requires knowledge of the mode Qn and of the dipole moment gradient in relation to this mode [8-IO]. it may be summarized as follows. In tk basis set of internal coordinates with S symmetry
3. Calculation conditions The study of the force field and of dipole moment derivatives was undertaken in the space of internal group coordinates (table I) with an initio SCF calcula316
* The variations of the cartesian coordinates used to calculate +/X3 are estimated in such a manner that the direction of the principal axis of the inertia teusor should be invariant. In these conditions the rotational contribution to the infrared intensities will be canceled [ 1 1 ] _
0 009~2614/83/0000-0000/$03.00
0 1983 North-Holland
CHEMICAL
Volume 99. number 4 Table 1 internal and symmetry
coordinates
-12 August 1983
PHYSICS LETTERS
for nitrosomethane
Assignment A’ symmetry
&
Ar
C-N
S,
AI
s3
3-‘“(Ad1
s4
6-‘R(2Ad,
N-O stretch CH3 sym. stretch CH3 asym. stretch CNO bend CH3 sym. deformation CH3 asym. deformation
%
S8
Ar 61n(Af31 + A& + AD3 - A~12 - Aa13 - A&23) 6-1n(2A~23 - Acrl2 - Acq3) 6-ln(2ADB1 - AS2 - A@3)
Sg
2-‘n(Adl
SlO
2e1R(Aa13
Sll
2-?A&
Sl2
AT a)
s6 S7
A” symmetry
a)AT = $(AT~,CNO
Ad,) - Adz - Ad3)
+ Ad2 +
Aq2)
torsion
[7]
Microwave [ 5 ]
l-471 1.198
1.480
C-Hr
1.079
1.094
C-H1
1.080
1.211
YN-O
VCH3s ?H3as 6CNO ‘CH3s 6CH3as ‘KH3 “CH3as
‘CH3as ‘_LCH3 T
with
ap/asi = (,=xq, (artr/asi12)1'2I ,
A, is expressed in P mol-l g’/2 cm-1 _
cmm2 and ap/aQ,
in esu
1.094
LCNO
114.9
113.2
LHICN
111.2
111.0
LHzCN
107.4
107.2
3. Results and discussion 3_1. Force constants The force constants obtained with the double-zetatype ab initio basis sets are generally well reflected. In
from which
A, = 0.305
?-N
+ AT~~CNO + AT~~CNO), I’ = dihedral.
4-31G
N-O
CH3 rock (out of plane)
- AP,)
Table 2 Calculated and observed structural parameters of nitrosomethane (bond length, .4; angle, deg)
C-N
CH3 rock (m plane)
CH3 asym. stretch CH3 asym. deformation
-Ad,) -
stretch
(F (W’Si)Lin)2,
order to account for both correlation and anharmonicit) effects, however, is it necessary to apply respective corrections of 10 and 20% to the valence diagonal and angular deformation terms [6] _ The crossed terms of the 317
CHEMICAL PHYSICS LETTERS
Volume 99, number 4
potential function were not modified. The force constants for the nitrosomethane molecule thus obtained are shown in table 3. As a result of the small number of experimental data, it was not possible to compare directly these values to a generalized valence force field. On the other hand, the Tzbie 3 Computed force field of nitrosomethane (in mdvn X-‘, mdsn rad-’ and mdvn ii rad-’ for z&etching, stret~hin~ben~mg and bending force constants respectively) Assignment
4-31G
4.040 0.916 0.169 -0.013 0.562 0.452 0.099 0.023 13.042 0.039 0.073 o-705 0.044 0.002 -0.106
_4ssigmnent
4-31G
5_3J5 -0.162 -0.014 0.183 0.142 1.626 0.035 0.034 -0.271 0.598 0.003 0.009 0.562 0.023
5.366 -0.108 -0.018 -0.091 -0.024 -0.009
4.997 -0.094 0.229 0.092 0.494 -0.329 -0.009
0.699
0.759 -0.006
0.014
12 August 1983
partial results of Turner and Cox 15] in the hypothesis of a simplified force field, as well as those proposed by Barnes et al. [4] for the study of the CNO group, may be valuable for discussionConcerning the main force constants of the CN and CNO vibrators, the results obtained (FCN = 4.04 mdyn A-’ and FCNo= I.626 mdyn A-f) are in excellent agree ment with those reported by Barnes et al. [4] (FCN =r. 3.80 mdyn A-’ and FCNo = 1.6 mdyn A-l) and are somewhat different from those reported by Turner and Cox [S] (FCN = 3.5 1 mdyn ,&-I and FCNo = 2.04 mdyn A-l)_ These values may also be compared to those determined by Pouchan et al. [ 121 for formaldoxime (FEND = 1.66 mdyn A-I) and by Shurvell et al. { 135 for tri~uoronitrosomethane (FC- = 3.616 mdyn a-t and In the latter case, the deFCNO = I.29 mdyn a-l). creased force constants may be explained by the elec~roneg3tivlty of the fluorine atom. Concerning the force constant of the NO vibrator, the calculated value (pNo = 13.042 mdyn A-1) is quite different from those calculated by Barnes et al. [4] (IO.80 mdyn A-l) and by Turner and Cox [S] (9.87 mdyn A-‘) for CHsNO. it is also different from that reported by Shurvell et al. 1131 (10.10 mdyn A-1) for the trifluorinated homologue. This disagreement, already reported for the formaldoxime molecule [ 12], is due primarily to the poor description of the atomic orbitals of the N atom in the 4-3 1G basis set [6]_ It should nonetheless be noted that an iterative catculalion on Fho based on the calcutated force field and reliable experimental frequencies generated a value of 10.88 mdyn A-’ for this constant, identical to that proposed by Barnes et al. [4]. For the other diagonal force constants associated with the movements of the methyl group, we note a satisfying agreement between our values and those calculated for the analogous vibrators of methyl formate [ 141 and acetaldehyde 1151, a molecule which is isoelectronic with nitrosomethaneThe constant F, of the torsional mode could not be experimentally determined. Our calculations result in a low value of 0.014 mdyn A-l rade2, included between that calculated for acetaldehyde (IS] (F,= 0.037 mdyn A-l) or methyl formate [ 141 (F,= 0.021mdyn A-l) and that determined for tri~uoronitrosomethane [ 131 (F,= 0.004mdyn A-l). All the crossed terms of the potential function
CHEMICAL
Volume 99, number 4
based on the analysis of the potential energy distribution, is shown in table 41 This spectroscopic study was complemented by the calculation of integrated absorption intensities. It enabled the infrared spectrum of nitrosomethane to be plotted theoretically and a priori (fig. 1). Several comments may be made concerning the comparison of our results and those of Barnes et al[4], obtained in an argon matrix. (i) All the calculated modes develop preferentially along one group coordinate, facilitating the assignment of the spectrum. (ii) The theoretical sequence of band positioning agrees with the experimental sequence. Even in the relatively restricted spectral range of methyl group valence vibrations, our calculations confirm the fact that the vCH,mode with A” symmetry is located between the vCHas and vCHa% modes with A’ symmetry and has a higher intensity that the latter two. (iii) Concerning the modes of the CHO group itself, it should be noted that there is excellent agreement between the wavenumbers calculated at 1564,896 and 541 cm-l to describe the movements of vNO, vcN and sCNo and the corresponding frequencies (1549,870 and 574 cm-r) observed in the matrix. This concordance in band position is corroborated by examining their intensity.
could not be obtained unambiguously by experimental means as a result of the insufficiency and imprecision of the latter. We may note the difference between the calculated value for FCN/N~ at 0916 mdyn A-l and the value proposed by Turner et al. [5] (1.5 1 mdyn A-l). In addition, there is a complete disagreement between our calculated value of 0.705 mdyn A-l for FN,,c.O and the negative value (-0.3 1 mdyn A-l) reported by Turner et al. [5] _ In the latter case, our results are consistent with those obtained with the homologous terms of the formaldoxime molecule [ 12]_ Similarly, there is perfect agreement between the value calculated for FCNIcNO (0.562 mdyn) and the corresponding values proposed in the case of fomraldoxime (0.526 mdyn) and trifluoronitrosomethane 1133 (0.3 16 mdyn). Among the other interaction force constants, supposed null in all experimental approaches, we noted relatively high values for the terms FCN16CHss, F CHO/r,,CH3 J FCH,as/r1CH3 and FsCH&lCH~3.2
Vibrational spectrum
The vibrational spectrum of nitrosomethane was constructed from frequencies calculated with the formalism of Wilson et al. [S]. The assignment performed, Table 4 Vibrational frequencies (cm-*)
calculated for nitrosomethane Yobs
A’
A"
12 August 1983
PHYSICS LETTERS
“cdc
a)
DEP b,
2991
3151
~CH,&92),
2901
3028
1549
1563
1410
1496
%H3s (‘l)- “CH3as cg) “NO (85 j, 5 &gas _ c6)* ‘CH$[ (51, VCN (21, SC,,0 6CH3as (841, +k:+ (61, YN(-J (8)
1348
1368
+H3s
967
1174
‘CH3n (701, SC,,0
870
896
VCN (86). pCH+s
574
541
&-NO (77),rCHa,,
2955
3052.
‘)CH3”s
1410
1440
sCHgas (86).‘CH3~
(13)
916
907
rCH31 (13),6CH,as
(36)
144
166
7 (831, “CH+
pCH3s
(8) (2)
cgl), 6CH3ar (5). PNO (2) (121, VCN (10). “NO (41, +H3s
(3)
(4). TCH311 t7) (22)
(100)
(1 l), 6CH+
(3). rlCH,
(3)
a) Corrected force COnSWitS (See text) - FNO = 10.88 mdyn 8;’ _ b) DEP calculated using corrected force field.
319
CHEhlICAL
full
0
1111
Wave
number
The theoretical study of the other modes is an indispensible support for the experimental study insofar as the ldtter alone cannot (i) distinguish the bands of 6~11~~ deformation modes with A’ and A” symmetry in the region of 1400 to 1.500 cmS1; (ii) subsequently assign a slightly intense but very wide band centered at around 1162 cm_1 ; (iii) finally, define the frequency of the torsional mode. The a priori study of the infrared spectnnn of nitrosomethane removes all ambiguity concerning these three points, primarily involving the modes associated with the methyl group. Our results seem to be valid, since the other two movements, SC& (A’) and rCH3 (A”), creating two bands with high and low intensity at I348 and 9 16 cm-l are shown perfectly by our ab initio calculations at 1368 and 907 cm-l_ In the region between 1400 and 1500 cm-l, our calculations predict two vibrations with different intensi-
width
at
fill
4511
Fig. 1. IR spectrum calculated
320
1Z AUfgISI lYiS5
PHYSICS LETTERS
half
maximum -1 20 cm
:
2211
(cm-‘)
for qitrosomethane.
ties, at 1496 and 1441 cm-l, assigned respectively to the A’ and A” asymmetric deformation modes of the methyl group. The difference between the two bands is greater than that observed or calculated for the same vibrations in acetaldehyde [15] and methyl formate [ 141. This enables us to predict the existence of two distinct bands which were not differentiated in the experimental analysis of Barnes et al. [4] as a result of the numerous lines arising from incomplete dimer elimination. The analysis of our calculations subsequently enables us to affirm that the wide band observed but not assigned by Barnes et al. [4] at 1162 cm-l is most probably attributable to the rCHJ rocking mode with A’ symmetry_ This vibration, calculated at 1175 cm-l, presents a small calculated intensity and may be compared to the frequencies chosen to describe this same mode in acetaldehyde [IS] (1114 cm-l) and in methyl
Volume 99,
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CHEMICAL PHYSICS LE-JTERS
formate [I43 (Ii68 cm-l). In these conditions, it is thus probable that the wavenumber of 967 cm-l adopted by Barnes et al. [4] to describe rocking movement is in error. The existence of this band is probably due to the presence of the trans dimer which absorbs to a great extent in this region. Finally, our calculations indicate that the torsional mode of the methyl group should appear at 166 cm-l in an experimentally unexplored spectral region. This value is in agreement with that estimated at 144 cm-l by Turner and Cox [S], based on the study of satellite lines in a microwave spectrum.
[I] W. Liittke, Z. Elektrochem. 61 (1957) 302. [2] R.N. Dixon and H.W. Kroto, Proc. Roy. Sot. A283 (1964) 423. [3] N.P. Emsting, J. Pfab and J. Riimelt, J. Chem. Sot. Faraday II 74 (1978) 2286.
I2 August 1983
[4] A.J. Barnes, HE. Hallam. S. Waring and J-R. Armstrong, J. Chem. Sot. Faraday II 72 (1976)-l_ [5] P-H. Turner and AP. Cox, f_ Chem. Sot. Faraday II 71 (1978) 533. [6] C_ Pouchan, A. Dargelos and M. Chaillet, J. Chlm. Phys. 75 (1978) 595. [7] P-D. Adeney, WJ_ Bouma, L_ Radom and W-R. Rodwell. J. Am. Chem. Sot. 12 (1980) 4069. [S] E-B. Wilson, J-C. Decius and P-C. Cross, Molecular vibrations (McGraw-Hill, New Y&k, 1955). [9] G.M. Barrow, Molecular spectroscopy (McGraw-Hill, New York, 1963). [lo] J. Overend, Infrared spectroscopy (Elsevier, Amsterdam, 1963). [ 1 l] TX Lakdar, M. Suard. E. Taillandier and G. Berthier, hiol. Phys. 36 (1978) 509_ [ 121 C. Pouchan, D. Liotard, A. Dargelos and M. ChaiUet, J. Ch-b-n. Phys. 11 (1976) 1046. [ 131 H-F. Shurvell, S-C. Dass and RD. Gordon, Can. J_ Chem. 52 (1974) 3149. [ 141 E.B. Marmar, C. Pouchan, A. Dargelos and M. Chaillet, J. Mol. StNCt. 57 (1979) 189. [IS] H. Hollenstein and Hs.H. Giinthard, Spectrochim. Acta 27A (1971) 2027.
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